The reason, truthfully, people are lazy and hate unpleasant experiences. I know I am. But, fear not! I have some tips to help make those healthy choices stick.
Find the Right Fit
If you don't like running it would be foolish to start an exercise regimen centered on running. Find an activity that you genuinely enjoy. Some people prefer yoga/Pilates to kickboxing/aerobics. But when it comes down to it the workout out the works the best and burns the most calories/fat is the one you can stick too.
Set yourself realistic goals, baby steps.... If your goal is to run a marathon in six months, that can be very overwhelming. To make it more manageable, break your long term goal (to run a marathon) into smaller short term goals. Start a couch to 5k program then graduate to a 10k, etc.... Also, the success of completely smaller goals will keep you motivated and moving forward to your long term goal.
Change ONE Behavior at a Time
Bad habits develop over a period of years and not overnight. So, choose one bad habit, for example drinking soda and change it for drinking more water instead. As, it becomes more of a habit to drink more water over soda, move on to another bad behavior to change that is moving towards your fitness/health goals.
Don't Go it Alone
Recruit a friend to exercise with or help you say no to that second helping of dessert. If you don't have a friend/family member, you can get a personal trainer, or find a support group. At many gyms there are running groups or fitness classes you can join if you feel you need the extra support or enjoy social workouts.
Stop Making Excuses
We have all fell victim to the "I'm too busy to workout out or don't have time to make healthy meals". Plan your activity 24 hours in advance and put it in your schedule book/calendar. If that means going to bed 30 minutes early so you can get up 30 minutes earlier to workout. Also, reducing our TV time to make time for a workout or planning a meal. (you know you watch too much tv, yeah you). It just comes down to making time for it and making it a priority.